BACKGROUND AND OBJECTIVES: Despite its
low incidence, aspiration of pulmonary gastric contents may have devastating
consequences. Esophageal sphincter function and protective airway reflexes decrease
caused by conscience depression, predisposes patients to this severe complication.
This article is a review of physiological aspects associated to gastroesophageal
reflux, as well as of the methods to prevent it.CONTENTS: Comments are made about the mechanisms involved in gastric
contents aspiration, its consequences and preventive methods, including recent
preoperative fasting guidelines developed after review of the literature, the
reasonable use of drugs acting on gastric pH and volume, and finally the effects
of different airway control methods on pulmonary aspiration prevention.CONCLUSIONS: Aspiration of pulmonary gastric contents, despite its low
frequency, demands special preventive care. Recently developed preoperative
fasting guidelines suggest shorter fasting periods especially for liquids, allowing
more comfort to patients and less risk of hypoglycemia and dehydration, without
increasing the incidence of perioperative pulmonary aspiration. The routine
use of drugs decreasing gastric acidity and volume seems to be indicated only
for poor risk patients. The best method to protect airways against aspiration
is still tracheal intubation. Other airway control methods have been adopted,
but their efficacy in preventing aspiration is lower, although representing
major alternatives in cases of intubation failure.

Recent studies have suggested that perioperative
pulmonary aspiration is an uncommon event however with devastating impact 1.
In 1946, Mendelson has already established a relationship between feeding and
aspiration of pulmonary gastric contents during labor under general anesthesia.
He has described two syndromes: the first would be the inhalation of solid food
leading to airways obstruction and death or massive atelectasis. The second,
named after him, would be liquid contents aspiration when laryngeal reflexes
are depressed by general anesthesia 2. These patients would develop
cyanosis, tachycardia and tachypnea. Mendelson has shown in rabbits that the
development of the syndrome would depend on the acid pH of aspired material
3.

In this article, mechanisms involved in regurgitation
and aspiration of pulmonary gastric contents are evaluated together with some
preventive methods.

INCIDENCE

A review of the literature suggests that the
incidence of perioperative pulmonary aspiration is relatively low and has suffered
minor changes in recent years. In 1986, a study of the Scandinavian Teaching
Hospital has observed that the incidence of aspiration varied from 0.7 to 4.7
per 10 thousand anesthesias 4. A publication 10 years later has observed
an incidence of 2.5 per 10 thousand in the Norwegian Hospital 5.

Mayo Clinic studies have observed similar incidence
in adults (3.1 per 10 thousand) 6 and children (3.8 per 10 thousand)
7. Another North-American study has observed a higher incidence in
children (10.2 per 10 thousand) 8.

Based on recently available data, pulmonary gastric
contents aspiration seems to be only slightly more frequent in children as compared
to adults. Children also seem to be less severely affected by aspiration 10.

MORBIDITY AND MORTALITY

Morbidity attributed to aspiration is characterized
by the presence of pulmonary infiltrates at chest X-rays, by the need for antibiotics
or bronchodilators and by ventilatory support duration 9 (Table
I).

In a Mayo Clinic study 6, 27% of pulmonary
aspiration patients needed ventilatory support for more than 24 hours.

Major retrospective studies performed in different
centers have shown mortality rates varying from zero to 4.5% 4-8.

In the UK, the "Confidential Enquirer into Maternal
Deaths" has presented accurate data on post-gastric content aspiration mortality
in obstetrics, from 1957 to 1998 11. For anesthesia-related deaths,
the proportion attributed to pulmonary aspiration has progressively decreased
from 50% to 65% fifty years ago, to up to 12% in recent 10 years. This decrease
is clearly the result of replacing general anesthesia by spinal blocks in obstetric
anesthesia 11.

Critical values for aspiration pneumonitis risk
derived from animal models are gastric contents volume above 0.4 ml.kg-1
and pH lower than 2.5 12. However, a study performed with cats has
shown that gastric volume necessary for spontaneous regurgitation was 20.8 mL.kg-1
13. Other investigators consider residual gastric volume between 0.8 and
8 mL.kg-1 as a risk factor for aspiration 13-16.

A critical review of the literature has revealed
that lower pH and higher gastric volume are major risk factors for pulmonary
aspiration in animal models. However, differences among species make human extrapolation
difficult and inaccurate 17. In addition, healthy patients with prolonged
preoperative fasting often have gastric volume above 0.4 mL.kg-1
and pH below 2.5 18-21.

So, in spite of evidences of direct relationship
between aspired volume and pneumonitis severity 14, the ratio between
gastric and aspired volume has been questioned and its validity needs further
studies 22.

In 1833, Beaumont has already recorded that after
fluid ingestion, stomach would empty in less than 1 hour, while emptying time
for solids was much longer 23. The dependence on gastric motility
for solid material emptying explains this difference, since in case of fluids
there is no such dependence 24. Water and fluids cross the stomach
very fast. Half of 500 ml isotonic saline solution bolus is emptied and/or absorbed
by the stomach in 12 minutes 25,26. Half of 750 ml bolus disappears
in 20 minutes 21,27,28 and 80% to 95% of ingested fluids disappear
in one hour 29,30.

However, gastric emptying time for solids varies
considerably.

Among types of ingested food, lipids emptying
is slower, proteins emptying is faster and carbohydrates emptying is intermediate
31. There is no absolute definition of solid food. In practical terms,
solid is every food in this state in the stomach 23. So, gelatin
is solid before ingestion, but is not in solid state in the stomach. On the
other hand, milk forms solid components in the stomach and takes hours to be
emptied.

Lower Esophageal Sphincter

Seemingly there is no true anatomic sphincter,
but muscle fibers of the junction of esophagus and stomach, which are disposed
in circle acting as a true sphincter.

The trend to regurgitation is not directly dependent
on lower esophageal sphincter pressure, but on the difference between this pressure
and intra-gastric pressure. Antiemetics, cholinergic drugs, succinylcholine
and antacids increase lower esophageal sphincter pressure, while anticholinergics,
thiopental, opioids and inhalational anesthetics decrease it. Atracurium, vecuronium,
ranitidine and cimetidine have no effect on lower esophageal sphincter 32.

Upper Esophageal Sphincter

Helps preventing aspiration by acting on the
transition exerted by the cricopharyngeal muscle between esophagus and hypopharynx.
During anesthesia, and even during normal sleep, there is a change in its function
and tone 33.

With the exception of ketamine, anesthetic drugs
decrease upper esophageal sphincter tone. In addition, patients receiving neuromuscular
blockers may be at risk of aspiration, even with TOF (train of four) of 0.7,
because upper esophageal sphincter tone and swallowing are still decreased 34-36.

Laryngeal Reflexes

Airway reflexes protect lungs against aspiration.
There are four well-defined reflexes 10: apnea with laryngospasm,
cough, expiration (forced expiration without preceding inspiration) and Spasmodic
Panting (superficial breathing with frequency of 60 movements per minute for
less than 10 seconds).

Two hours after outpatient general anesthesia
recovery, upper airway reflexes sensitivity has not returned to normal 37.
This decrease in reflexes seems to be present not only in the intraoperative
period, but also in premedicated patients and in the postoperative period, probably
for a long than expected period. In addition, elderly people have less active
airway reflexes and should be considered at increased risk for aspiration 37-40.

The objective of preoperative fasting is to decrease
gastric contents regurgitation risk and degree, thus preventing pulmonary aspiration
and its consequences.

The old orientation "nothing by mouth after midnight"
has been replaced by shorter preoperative fasting periods. There are several
benefits when patients, especially children, ingest fluids before anesthesia,
including higher satisfaction and less irritability, gastric pH increase, decreased
risk for lipolysis hypoglycemia and dehydration 15,21,44-46.

Studies in different centers involving children
ingesting different types of clear fluids (water, tea, coffee, fruit juice without
pulp, all without alcohol and with little sugar) in variable volumes have concluded
that fluid ingestion, without volume limitation, may be safely allowed 2 hours
or more after surgery 23,47.

In general, stomach acid secretion production
rate is 0.6 mL.kg-1.h-1 21, but it may reach 500 ml.h-1
with fasting and hunger 19,30,48. It has been observed in many cases
a decrease in gastric pH with increased preoperative fasting duration 49.
Anxiety is an emotional stimulation able to increase HCI production, similarly
to the cephalic phase of gastric secretion 17,48,50,51, which explains
increased volume and decreased gastric pH after prolonged fasting 17,27,49,52,53.
Increased gastric pH in patients receiving fluids 2 to 3 hours before surgery
could be result of the dilution of acid secretions and/or decrease in their
production by the decrease in anxiety and hunger levels. Gastric volume decrease
in patients receiving fluids few hours before surgery could be due to gastric
motility stimulation by cold fluid entrance and/or physical stomach distension
15,17,27,54.

In spite of all knowledge obtained to date, it
is not possible to surely predict gastric contents. Healthy patients under prolonged
fasting may, during surgery, present vomiting with contents of previous day
meal. Others may present hypoglycemia, dehydration and irritability. However
it seems reasonable to conclude that unrestricted ingestion of clear fluids
for healthy patients 2 hours or more before surgery has a more than acceptable
risk/benefit ratio 7,8,11,21,55,56.

The American Society of Anesthesiologists, through
the ASA Task Force on Preoperative Fasting 57, has developed practical
guidelines for preoperative fasting and for the use of drugs involved in decreasing
gastric volume and acidity. Based on an extensive literature review, guidelines
are aimed at healthy patients of all ages submitted to elective procedures,
and do not include patients at increased aspiration risk. Such recommendations
may be adopted, modified or rejected according to individual clinical needs
and are subject to periodic reviews according to the evolution of knowledge
on the subject.

Clear fluids (water, tea, coffee, fruit juice
without pulp, all without alcohol and with little sugar): 2 hours fasting
for all ages;

Breast milk: 4 hours fasting for neonates
and infants;

Light diet (tea and toasts) and non breast
milk: up to 6 hours fasting is accepted for children and adults;

Pediatric formula: 6 hours fasting for neonates
and infants;

Solids: 8 hours fasting for children and
adults.

Many investigators have studied the implementation
of new preoperative fasting guidelines to replace the old recommendation "nothing
by mouth after midnight". Ferrari 58 has evaluated 51 institutions
in the USA and Canada and has concluded that the approach preconized by the
American Society of Anesthesiologists represents most pediatric institutions
in North America. Investigations in the UK, Scotland, Germany and Norway have
also shown that more flexible preoperative fasting measures have been adopted
by most institutions in those countries without increasing complication rates
59-62.

A recent study with healthy non obese pregnant
women who were not in labor, has not found difference in gastric contents after
the ingestion of 300 ml of water as compared to prolonged fasting 64.

The American Society of Anesthesiologists Task
Force on Obstetrical Anesthesia 65 recommends that moderate clear
fluid ingestion may be allowed for pregnant women in uncomplicated labor. When
there is other associated risk factor (diabetes, morbid obesity, difficult airway),
or patients are at high risk to evolve to C-section, fluid restriction should
be determined in a case-by-case basis. As to solids, the committee agrees that
8 hours or more for elective C-sections is the most adequate period.

Labor patients should not ingest solids.

Mechanisms involved in increased pulmonary aspiration
risk in obese patients do not include increase in gastric contents because it
is not higher as compared to the volume of non-obese patients. Other factors,
such as intra-gastric, abdominal and lower esophageal sphincter pressures probably
play a more important role in aspiration pathophysiology of obese patients 66.

According to the Norwegian National Consensus,
preanesthetic medication may be ingested by adults with 150 ml of water up to
one hour before anesthesia. The same is true for children but with the limitation
of 75 ml of water 23.

Gastric Acidity Decrease

Many clinical studies have been performed in
Anesthesia about the use of H2 receptor antagonists and proton pump
blockers in healthy patients to evaluate their effects on gastric volume and
pH 67.

A single 150 mg ranitidine dose few hours before
anesthetic induction significantly increases gastric pH, in addition to decreasing
its volume 67-69.

Protons pump blockers prescription, however,
requires the understanding of its pharmacology. Studies have shown that such
drugs are more effective if administered in two successive doses: one the night
before and the other in the morning of anesthesia 67-69.

Although being possible to show that these drugs
increase gastric pH and decrease its volume, the ASA Task Force considers that
there are no evidences supporting their routine use in healthy patients, but
only in risk patients, since there is no proven decrease in aspiration, morbidity
or mortality rate with the use of such drugs in healthy patients 57.

The efficacy of preoperative antacids to decrease
gastric acidity is proven but there are no evidences that these solutions act
on gastric volume. Particulate antacids may increase the risk for pulmonary
injury in case of aspiration and should be avoided. So, preoperative antacids
(non-particulate such as sodium citrate) should only be prescribed to risk patients
57.

Anti-emetics (e.g.: droperidol and ondansetron)
are also solely indicated for patients at higher risk for aspiration 57.

Nasogastric Tube

Nasogastric tubes are commonly inserted in risk
patients before anesthesia, aiming at stomach emptying. However, it seems that
upper and lower esophageal sphincters function is impaired as compared to the
function in patients without nasogastric tubes 42,70. Sellick recommends
in his original study 42 that the tube should be removed before anesthetic
induction. However, two studies with cadavers have shown that the efficacy of
the Sellilck's maneuver is not decreased with its presence 71,72.
So, the tube would act as a safe passage for gastric contents when effective
cricoid cartilage compression is applied. Based on these data, Smith 9
suggests that the tube should be left in situ during rapid sequence induction
associated to Sellick's maneuver.

There are no significant differences in pulmonary
aspiration rate when different tube sizes are used 73.

A cuff associated to the tube has been successfully
used aiming at occluding the cardia, thus gastroesophageal reflux 74.
This tube with gastric cuff has been studied together with laryngeal mask. It
has been observed that the tube does not interfere with laryngeal mask insertion
and that this would not prevent tube insertion. So, this association represents
a good option for difficult airways manipulation in patients at risk for aspiration
75.

Cricoid Cartilage Pressure

Cricoid cartilage pressure (Sellick's maneuver)
has become universal practice during anesthetic induction in patients with potentially
full stomach 76. Such maneuver, when correctly performed, prevents
gastric inflation in children 77 and adults 78,79, in
addition to increasing upper esophageal sphincter tone 80. Lower
esophageal sphincter tone decreases with such compression 81, suggesting
the presence of pharyngeal mechanoreceptors promoting reflex relaxation of this
sphincter. This effect, however, does not seem to cause gastroesophageal reflux
82.

The incorrect application of the maneuver may
deform cricoid cartilage, close vocal cords and impair ventilation, especially
in women 83. Strength applied should be sufficient to prevent aspiration,
but not so strong to cause airway obstruction or allow for esophageal rupture
in case of vomiting. Cephalad and backward direction of the strength applied
seems to improve laryngoscopic visualization 84.

A major scenario to be considered is tracheal
intubation failure in patients with full stomach under cricoid cartilage compression.
In this situation, a laryngeal mask may help ventilation and oxygenation, but
studies have shown that Sellick's maneuver prevents its correct positioning
85,86. So, if laryngeal mask is used during tracheal intubation failure,
it may be necessary to temporarily interrupt the maneuver to help ventilation,
oxygenation and intubation through the laryngeal mask. Interruption seems to
be a reasonable option since cricoid cartilage compression may become ineffective
after few minutes of application 87.

EFFECTS OF DIFFERENT AIRWAY CONTROL METHODS
ON ASPIRATION RISK

Tracheal Intubation

Tracheal intubation is the most effective method
to protect airways in anesthetized patients. However, some studies performed
with intubated patients in intensive care units, have shown that high volume
and low pressure cuffs have allowed the leakage of stain to the trachea 88,89.
Mechanism involved might be microaspiration through the glottis by small channels
between the cuff and tracheal mucosa 90. The lubrication of cuffs
with gel has significantly decreased leakage 90,91.

Laryngeal Mask

Laryngeal mask is associated to decrease in pressure
barrier represented by lower esophageal sphincter 92. A study of
patients under general anesthesia with positive pressure ventilation has shown
that in the group using laryngeal mask there has been a significant increase
in gastroesophageal reflux as compared to the group using tracheal tube with
cuff 93. There is also reflux in patients under spontaneous ventilation
and laryngeal mask 94. In fact, there are no significant differences
in regurgitation rate between groups spontaneously ventilating or under positive
pressure ventilation 95,96. Time for laryngeal mask removal at the
end of general anesthesia seems to influence reflux rate since those who had
the mask removed when able to spontaneously open their mouths had a lower incidence
of reflux as compared to those who had their masks removed before consciousness
recovery 97.

The development of a new laryngeal mask model,
ProSeal (Laryngeal Mask Company, Henley on Thames, UK), allows the introduction
of orogastric tube through an esophageal opening for better gastric contents
drainage, in addition to a better gastric inflation blockade during positive
pressure ventilation. A recent study has shown that ProSeal is more effective
than traditional laryngeal masks to prevent reflux, what makes it a good option
for patients at aspiration risk and failed tracheal intubation 98,99.

COPA and Combitube

"COPA" (Cuffed Oropharyngeal Airway) is even
less effective than the laryngeal mask to prevent regurgitation 100,
thus not being indicated for patients at aspiration risk.

Combitube, although protecting against regurgitation
and allowing gastric contents drainage, requires a trained professional, since
its use is associated to complications such as sore throat, dysphagia and hematomas.
So, in emergency situations when airway protection is needed, combitube benefits
should be counterbalanced with possible associated complications 9.

OTHER PREVENTIVE GASTRIC ASPIRATION MEASURES

Forecasting of Difficult Tracheal Intubation

The first step of difficult airway algorithm,
developed by the American Society of Anesthesiologists is the identification
of patients with potential difficult tracheal intubation 101. In
some cases, such as facial trauma, some congenital syndromes or morbid obesity,
difficulty may be easily identified. However, in less evident cases, intubation
difficulty may only appear after anesthetic induction, when protective reflexes
are abolished, and may expose patients to aspiration risks. So, the forecasting
of difficult intubation may be done through some parameters, such as Mallampati's
test 102, thyroginean distance or other tests available in the literature
103.

Patients Positioning 3

Patients at risk for aspiration should be placed
in the horizontal position with elevated dorsum approximately 30º with relation
to the rest of the body. This position will prevent regurgitation, but if it
happens, table position should be immediately changed so that the head is positioned
below the trunk, thus preventing aspiration.

TRACHEAL INTUBATION WITH AWAKEN PATIENT OR
UNDER RAPID SEQUENCE INDUCTION

Awaken Patient

Once a potential difficult airway is identified,
the safest intubation method is with the patient awaken and under spontaneous
ventilation 103.

Tracheal intubation may be performed after mild
sedation, maintaining airways protective reflexes. Antisialogogues, such as
atropine or scopolamine, should be administered and oropharynx should be topically
anesthetized with spray lidocaine (10%). Glossopharyngeal and upper laryngeal
nerves blockade might be useful for those mastering the technique 101.

Rapid Sequence

Rapid sequence induction traditionally involves
preparing the equipment, which should include, in addition to tracheal intubation
material, one aspirator, one backup laryngoscope and equipment for intubation
failure (laryngeal mask, fastrack, fibroscope); 100% oxygen administration under
facial mask for 1 minute; short onset and duration intravenous opioid and anesthetics
(e.g.: propofol and alfentanil) and finally an also short onset and duration
neuromuscular blocker 3,104.

Succinylcholine has been used for 50 years by
most anesthesiologists as the neuromuscular blocker of choice to intubate full
stomach patients, since it is the drug which best meets the above-mentioned
criteria. However, complications such as malignant hyperthermia, fatal hyperkalemia,
bradyarhythmias, intragastric and intraocular pressure increase have encouraged
the search for adepolarizing neuromuscular blockers with shorter onset and duration,
without succinylcholine side-effects, hence, the introduction of rocuronium
as an alternative for patients under pulmonary aspiration risk. To assure onset
and intubation conditions similar to succinylcholine, preconized dose is 1 mg.kg-1
or more, which implies longer duration, impairing its use for short procedures
or those with possibilities of difficult intubation 105-108.

Succinylcholine or rocuronium, intubation with
awaken patient, fibroscopy etc., all have advantages and disadvantages. The
anesthesiologist should decide the best alternative on a case-by-case basis
109.

ASPIRATION PNEUMONITIS

Pulmonary aspiration is defined as inhalation
of oropharynx or stomach contents through the larynx to low respiratory tract
110,111. Several pulmonary syndromes may be present after aspiration,
depending on quantity and nature of the material. Aspiration pneumonitis (Mendelson's
syndrome) is a chemical injury caused by sterile gastric contents aspiration,
while aspiration pneumonia is an infection caused by inhalation of colonized
material especially coming from oropharynx. Other syndromes include mechanical
airway obstruction, pulmonary abscess and chronic interstitial fibrosis 110,111.

Treatment includes oxygen administration and/or
ventilatory support, when necessary. Preventive antibiotics are not indicated
because they may turn patient susceptible to secondary infection by more resistant
organisms. Antibiotics are indicated for patients with aspiration pneumonitis
not resolved 48 hours after aspiration 113. Steroids are not recommended
because multicentric randomized and controlled studies have failed in proving
their benefits 114,115.

Bronchoscopy should be performed in patients
under suspicion of solid material aspiration causing airway obstruction. Pulmonary
washing under direct view is also indicated for solid material aspiration 3.

CONCLUSIONS

Pulmonary gastric contents aspiration, although
uncommon, requires special preventive care. Recently developed preoperative
fasting guidelines suggest shorter fasting periods, especially for fluids, allowing
more comfort to patients and less risk for hypoglycemia and dehydration, without
increasing the incidence of perioperative pulmonary aspiration. The routine
use of drugs decreasing gastric acidity and volume seems to be indicated only
for risk patients. The best way to protect airways against aspiration is still
tracheal intubation. Other airways maintenance methods have been used but their
efficacy in preventing aspiration is still lower, although representing major
alternatives for tracheal intubation failure. Cricoid cartilage compression,
rapid sequence induction or induction with awaken patient, in addition to patient's
positioning, play an important role in preventing pulmonary aspiration.

11. Department of Health and Social Security. Deaths due to complications of anaesthesia. In: Report on confidential enquirer into maternal deaths in England and Wales. London: Her Majesty's Stationary Office, 1957-1998. [ Links ]